This invention relates to a circular slide rule suitable for use in determining the electrolyte deficiency in the extracellular space from the measured electrolyte deficiency per liter serum, from the body weight and the body size.
It is known that the electrolyte deficiency in the extracellular space (hereinafter generally referred to as " ECR" ) of infants, children and adults can be determined from three known values, namely the change of electrolyte concentration in the serum in mVal/liter, the body weight in kg and the body height in cm. The theoretical basis for this determination is found in the linear dependence of ECR on the body surface area (hereinafter generally referred to as " KO" ) which is independent of age, sex and constitution and is expressed by the formula: EQU ECR.sub.(kg) = 6.04 .times. KO.sub.(m.sbsp.2)
and by the functional dependence of the body surface area (m.sup.2) on the body weight (kg) and the body height (cm) in accordance with the formula: EQU KO.sub.(cm.sbsp.2) = kg.sup.0.425 .times. cm.sup.0.725 .times. 71.84
Clinical balancing in specific electrolyte therapy is usually performed by calculating the required amount of electrolyte to be supplied from the product:
CHANGE OF SERUM CONCENTRATION IN MVal/liter.times. KG(kg)/5 The quotient KG (kg)/5 in this case is characterized as the magnitude of ECR (kg). However, even in adults, this formula will provide only a very rough approximation to the actual value of ECR. Errors of up to 100% are likely in children and even more in the case of infants.
Measurements have shown that the ECR value is approximately 39% by weight at birth and approximately 27% by weight for a body weight of 10 kg. In adults, the ECR value is reduced to approximately 15% by weight for a body weight of 100 kg. The wrong treatment will be frequently applied when taking into account the usual "mean value" of 20% by weight of ECR proportion of the body weight. To this extent, various tables and mechanical calculating aids based on the above-stated theoretical basis and in use in various embodiments are not suitable for providing satisfactory therapeutic results.
It is known that there is a linear relationship between the surface area of the body and the magnitude of the ECR value. The amounts of water and electrolyte required to be supplied for daily use are, therefore, usually stated in relation to the body surface area by specialists for electrolyte therapy. New investigations in recent years have shown that, by contrast to the intracellular space, the linear function of the ECR to the body surface area remains constant not only at different ages but also with different sexes and a different body constitution. It is, therefore, sensible to relate the calculation of the electrolyte efficiency in the ECR to the body surface area and not to the body weight.
Special nomograms for children and adults were developed some considerable time ago, from which nomograms the body surface area can be obtained in dependence on the body weight and the body height. For accurate electrolyte therapy, it is necessary to multiply these values of body surface area obtained from two different nomograms with the values of electrolyte deficiency per liter of serum as defined in the laboratory and with the conversion factor of the body surface area/ECR relationship either by means of an additionally required conventional slide rule or by a written calculation.
The entire operation of reading the nomograms followed by further conversion and multiplication of the values obtained with due reference to the measured values is very time-consuming and above all very awkward.
It is an object of the present invention to provide a circular slide rule for use in infusion therapy, which circular slide rule can be easily handled and used to rapidly supply precise values of electrolyte deficiency for infusion therapy. The circular slide rule is to be of simple and handy construction and its circumference should be sufficiently small to enable the physician to carry the same readily in a smock pocket or the like.
According to the first aspect of the present invention there is provided a circular slide rule suitable for use in determining the electrolyte deficiency in the extracellular space from the measured electrolyte deficiency per volume of serum, body weight and body height, which circular slide rule comprises a basic disc, a centrally rotatable first rotary disc supported on one side of the basic disc but having a smaller diameter than the basic disc, a centrally rotatable second rotary disc supported on the side of the basic disc opposite to that of the first rotary disc but having a diameter smaller than that of the basic disc, and substantially straight cursors, each being rotatably supported on one of the rotary discs relative to the appropriate rotary disc and with respect to the basic disc, each said cursor extending at least to the circumference of the basic disc.
According to the second aspect of the present invention there is provided a method of determining the electrolyte deficiency in the extracellular space, which method comprises determining the electrolyte from the measured electrolyte deficiency per volume of serum, body weight and body height, utilizing a circular slide rule which comprises a basic disc, a centrally rotatable first rotary disc supported on one side of the basic disc but having a smaller diameter than the basic disc, a centrally rotatable second rotary disc supported on the side of the basic disc opposite to that of the first rotary disc but having a diameter smaller than that of the basic disc, and substantially straight cursors, each being rotatably supported on one of the rotary discs relative to the appropriate rotary disc and with respect to the basic disc, each said cursor extending at least to the circumference of the basic disc.
In a preferred embodiment of the invention, the cursors are provided with an optical radial scale marking.
Further features which are preferred in the present invention are disclosed hereinafter. It has been found particularly advantageous that each of the rotating discs is provided with a body weight and body height scale and an associated scale of change of electrolyte concentration and the basic disc is provided on both sides with a scale indicating the magnitude of the extracellular space (ECR) and a scale indicating the required infusion therapeutic electrolyte deficiency.
To this end, one of the rotary discs can for example cover a body weight range of from 2.5 to 25 kg, a body height range of from 45 to 125 cm and a .DELTA.mVal range of from 1 to 10 mVal and the other rotary disc can for example cover a body weight range of from 25 to 100 kg, a body height range of from 125 to 200 cm and a .DELTA.mVal range of from 1 to 10 mVal, and it is also proposed that the scales disposed on one of the rotary discs can have different lengths for identical radii and complement each other to form a total scale of 360.degree. .
Another embodiment of the invention is provided with scale lengths of the body weight (kg) and of the body height (cm) on each rotary disc which are related to each other as the differences between the logarithms of the appropriate scale starting values and scale end values of the body weight scales (kg) on the one hand and of the body height scales on the other hand on the circular scale. It can also be provided that the circumferential edge of the basic disc is provided with the scale indicating the required infusion-therapeutic electrolyte deficiency and, within the scale but on a smaller radius, the ECR scale. To this end, it is advantageous if the scale length of the ECR scale is proportional to the sum of the scale lengths for body weight and body height with due reference to the radius which is enlarged relative to the total scales of the rotary disc, and it can be provided more particularly that the scale divisions of the ECR scale are selected so that the sum of distances on the kg (body weight) scale and the cm (body height) scale provides the associated value on the ECR scale when based on the formula: EQU log ECR.sub.(kg) = log 0.1009888+ 0.5 .times. log "kg" + 0.5 .times. log "cm."
In the preferred embodiment of the present invention, the .DELTA.mVal scales of the rotary discs have a length which is such that their linear addition to the length of the ECR scale of the basic disc results in a full circle circumference, correspond to the mVal scale of the required infusion-therapeutic electrolyte deficiency and the scale lengths of the .DELTA.mVal scales and of the ECR scales are related to each other as the differences of the logarithms of the appropriate scale beginning values and scale end values on the .DELTA.mVal scales on the one hand, and on the ECR scales on the other hand when referred to a radius of identical size.